1. Field of the Invention
The present invention relates to a battery having a structure with a power generating element sealed in a battery case of a resin film and a lead externally extended.
2. Description of the Related Art
Conventionally, some batteries such as a lithium ion battery incorporated in a compact electronic appliance such as a portable telephone, a portable personal computer, a portable video camera, etc. have a structure with a power generating element sealed in a flat battery case of a resin film and a lead externally extended. The above resin film of the battery case is formed of a laminated film of a metallic foil and a resin film. The laminated resin film is formed to be a cylinder, and a lead is externally extended from one of the openings of the cylinder. The battery case is hermetically sealed in such a manner that both edges of the one opening are deposited to each other and both edges of the other opening of the cylindrical laminated film are also deposited to each other. At the extended portion of the lead, the internal faces of the film is kept in contact with the lead, and at the other portion thereof, the internal faces of the film are kept in contact with each other. In this way, the hermeticity of the battery is maintained.
The battery having such a structure can be produced as follows.
First, a predetermined portion of the lead is covered with resin, and the lead is connected to the electrode of a power generating element. The lead is wrapped with the film serving as a material of the battery case so that it is partially exposed. In this case, the power generating element and lead must be positioned so that the edges of the film and the covering resin cross each other. At the one end of the lead, the edges of the film are deposited to each other, and the edges of the film and the covering resin are deposited to each other. On the other hand, at the other end of the lead, with the edges of the film being opened, electrolyte is injected and thereafter they are deposited to each other.
However, when the edges of the film and the covering resin are deposited to each other, if the quantity of heat to be applied is insufficient, they are not brought into intimate contact with each other, thus reducing the hermeticity between the lead and resin. Inversely, if heat is excessively applied, the portion of the resin on the side of a heat source will be deformed before the resin is deposited to the lead. Thus, the hermeticity between the battery case and the resin will be reduced and the resin will be deposited to the heat source. This leads to reduction of a production yield. Further, if the resin is molten excessively, the lead may be brought into contact with the metallic foil constituting the laminate film so that a lead of a positive electrode and another lead of a negative electrode are short-circuited to each other through the metallic foil.
Further, when the power generating element and the lead are wrapped with the film, it is not easy to position them accurately so that they are misaligned with each other. Then, the film and the covering resin of the lead are not in intimate contact with each other so that the hermeticity therebetween may be lowered. In addition, the end of the power generating element is excessively pressed by the battery case so that the power generating element may be deformed and the lead may be broken.
It is an object of the present invention to provide a battery which can suitably perform film deposition to provide high hermeticity and provide excellent insulation between leads with positive and negative electrodes.
It is another object of the present invention to provide a battery which can position a lead and a film accurately to provide high degree of hermeticity.
The battery according to the invention has a structure in which a power generating element is housed in a battery case of a resin film, a prescribed portion of a lead is covered with a covering resin, and the power generating element is sealed by deposition between edges of the film constituting the battery case and between said covering resin and the edges of the film. The softening point of the deposited portion of the covering resin to the laminate film, i.e. the outer portion of the covering resin is made higher than that of the inner portion thereof.
In the battery according to the invention, the deposited portion of the covering resin covering the lead to the film edges has a high softening point. Therefore, the covering portion of the resin with the film edges is difficult to deform at a high temperature during thermal deposition and hence a predetermined thickness thereof is assured. Excellent hermeticity of the deposited portion can be obtained between the covering resin and film edges so that the leads of a positive electrode and a negative electrode are insulated from each other by the covering resin.
An example of the invention is to crosslink-couple the outer portion of said covering resin.
The resin is crosslink-coupled preferably by irradiation of electron beams rather than using a crosslinking agent. This is based on the following reason. In the case of using the crosslinking agent, the resin must be crosslinked at a higher temperature than the melting point and further supported by a support. On the other hand, in the case of the irradiation of electron beams, the resin can be crosslink-coupled at both low and high temperatures and does not require the support.
The covering resin may be composed of a plurality of layers stacked in its thickness direction so that the outermost layer has a higher softening point than the innermost layer. In this configuration, in order to bring the covering resin into intimate contact with the lead, with a heat source being in pressure-contact with the outermost layer of the covering resin, heat and pressure are applied toward the lead. Then, the temperature of the innermost layer of the resin reaches a softening point before that of the outermost layer of the resin does. As a result, the innermost layer is deposited to the lead. Thus, the resin on the side of the heat source does not become deformed and the resin is also not deposited to the heat source.
The specific technique of making the outermost layer have a higher softening point than the innermost layer includes to irradiate the outermost layer of the laminated covering resin composed of a plurality of resin films with electron beams, to laminate a plurality of resin films including the outermost layer crosslinked by irradiation of electron beams, etc.
The covering resin is preferably formed of denatured polyolefine. Its more preferable example is acid-denatured polyethylene. When the covering resin is acid-denatured polyethylene, whether or not it has been crosslink-coupled can be known by checking the gel percent of the insoluble part when it is dissolved in xylene or checking the thermal deformation.
The crosslinking degree is preferably 20%-95% (inclusive), more preferably, 30 to 80%, in terms of the gel percent. The reason thereof is follows. Namely, if the covering resin is insufficiently crosslinked, it will be dissolved so that its shape cannot be maintained and will be reduced in thickness. Inversely, if the covering resin is excessively crosslinked, the deposition of the resin to the film becomes difficult. In both cases, the hermeticity is reduced.
In the case of employing the thermal deformation, for example, with certain load applied to the resin, its temperature is boosted to 200xc2x0 C. The thickness at this time is measured. If the thickness is in a range of 50-60% of that before the temperature is boosted, it may be admitted that the resin is crosslink-coupled.
The difference between the softening points of the innermost and outermost layers is preferably 5xc2x0 C.-100xc2x0 C., more preferably, 5xc2x0 C.-30xc2x0 C., (inclusive) when the covering resin is composed of only these two layers and the thickness of each of these layers is 10-200 xcexcm. This is based on the following reason. If the difference is less than 5xc2x0 C., the function of the resin described above can not be sufficiently shown. Inversely, if the difference exceeds 100xc2x0 C., even if the innermost layer is brought into intimate contact with the lead, the outermost layer, whose softening point is too high, cannot be brought into contact with the battery case. Even if the outermost layer is in a state capable of being brought into contact with the battery case, the viscosity of the innermost layer is lowered so that the resin flows to a point far from the lead. As a result, the contact strength between the resin and the lead is lowered.
The covering resin is preferably colored. Then, the position of the resin can be visually inspected or optically detected. Therefore, the position of the lead covered with the covering resin can be accurately known. Thus, the lead can be caused to cross the film edges. In addition, when the power generating element and lead are wrapped with the film, the positions of them can be determined accurately. Accordingly, the power generating element can be covered with the film at the edges thereof without being pressurized.
If the resin is colored, the positioning can be automated using optical detection.